Susan CartwrightOur Evolving Universe1 Planets and life n
Successful detections of extrasolar giant planets suggests that
planetary systems may be fairly common l could we detect Earth-
like planets? l is it likely that such planets would have life? l
how would we know?
Slide 2
Susan CartwrightOur Evolving Universe2 Detection of Earth-like
planets n Doppler shift technique will not work n Two possible
strategies l transit detection WASP ground-based (now) CoRoT
space-based (now) Kepler (NASA) within 5 years? l direct imaging
requires space-based interferometer multiple telescopes acting as
one Darwin, TPF >10 years n One system of Earth-mass planets has
been found l around a pulsar! not well understood, but clearly not
really Earth-like
Slide 3
Susan CartwrightOur Evolving Universe3 Life on Earth: the
fossil record n Oldest rocks ~3.8 Gyr old n Oldest fossils ~3.5 Gyr
l bacteria n Oldest eukaryotes (nucleated cells) ~2 Gyr l coincide
with rise of atmospheric oxygen n Oldest multicellular organisms
~550 Myr n Early hominids ~5 Myr Fossil stromatolites, W. Australia
Shark Bay Dickinsonia, Vendian fossil
Slide 4
Susan CartwrightOur Evolving Universe4 Life on Earth:
implications n Life appears very quickly l easy process? n but
>80% of history of life consists of single-celled organisms l
becoming multicellular is hard? l on the other hand, it evolved
several times... n and intelligence appears only in last 0.001% of
lifes timeline l intelligence is hard? Main elements used by life
are very abundant Organic (carbon-based) compounds form easily in
early-Earth conditions Route from there to DNA+protein organisms
not well understood RNA world first? Liquid water looks essential
Main elements used by life are very abundant Organic (carbon-based)
compounds form easily in early-Earth conditions Route from there to
DNA+protein organisms not well understood RNA world first? Liquid
water looks essential Arguing from a single example is
intrinsically unsafe and there is clear selection bias (we
exist!)
Slide 5
Susan CartwrightOur Evolving Universe5 Life elsewhere: the
solar system n Venus looks superficially very similar to Earth l
but runaway greenhouse effect leads to surface temperature of 745 K
(472C) n Mars is too small to keep its atmosphere l evidence for
running water early in its history l life might have evolved, but
not complex life finding bacterial fossils would be major step:
confirm that originating life is easy n Europa is heated by tidal
forces l icy crust probably overlies liquid water ocean Magellan
HST Galileo
Slide 6
Susan CartwrightOur Evolving Universe6 Life elsewhere: other
stars n Criteria for candidate stars: l long enough lifetime life
unlikely to evolve in 10-million- year lifetime of 10-solar-mass
star star of 1.7 M sun has 2 Gyr main sequence llfetime classes
FGKM are OK l high heavy element content evidence suggests this is
required for planet formation not stars in halo l stable star,
stable orbits not close binary? not very low mass stars (often
unstable flare stars)? n Many stars satisfy these criteria
Slide 7
Susan CartwrightOur Evolving Universe7 Habitable zones n Liquid
water probably essential for life l candidate planets must be in
appropriate temperature range l temperature basically determined by
distance from star l habitable zone should allow for stellar
evolution u continuously habitable zone also affected by size of
planet l problem for low mass stars: tidal locking planets face
star extremes of temperature J.F. Kasting et al.
Slide 8
Susan CartwrightOur Evolving Universe8 Planets in habitable
zones n Some observed extrasolar planets are in habitable zone l
remember these are (probably gas) giants l remember detection
method biased in favour of planets near stars certainly no evidence
against habitable planets also possibility of rocky satellites of
gas giants (Ganymede, Callisto and Titan are Mercury-sized) J.F.
Kasting et al. Extrasolar planets
Slide 9
Susan CartwrightOur Evolving Universe9 Signs of life n Oxygen
is highly reactive l not stable in atmosphere: maintained by plants
l earliest fossils already photosynthesising oxygen in atmosphere
good indicator of life even in early stages l spectroscopic
detection possible in infra-red to reduce background from star good
for 3-atom molecules detect CO 2 (atmosphere), H 2 O (oceans), O 3
(life) Simulated image and spectrum from DARWIN homepage
Slide 10
Susan CartwrightOur Evolving Universe10 Life like us? n How
probable is evolution of intelligent organisms with technological
civilisation? l Drake equation (Frank Drake, SETI pioneer) Number
of communicating civilisations = rate of formation of suitable
stars x fraction of these stars with planets x number of Earth-like
planets per system x fraction of such planets which develop life x
fraction of life-bearing planets evolving intelligence x fraction
of intelligent species developing technology x average lifetime of
such a civilisation information obtainable on factors marked last
three very difficult to estimate rough guess: assume 10 stars/year
and all fractions = 0.1; then number of civilisations = average
lifetime/10000 known high? ? high? controversial ? ????
Slide 11
Susan CartwrightOur Evolving Universe11
earchforxtra-errestrialntelligence S earch for E xtra- T errestrial
I ntelligence n Drake equation suggests other civilisations may
exist l only way to confirm this is direct detection l obvious
method of communication is radio galaxy fairly transparent to
appropriate wavelengths travels at speed of light fairly easy to
send and detect l radio covers a wide wavelength range! need to
guess appropriate wavelength 21 cm? (hydrogen) microwaves? (cosmic
background) l would we recognise a signal? probably:
information-carrying signal very different from natural sources l
would we be able to decode it? much more problematic SETI@home:
Berkeley programme using Arecibo telescope at 21 cm
Slide 12
Susan CartwrightOur Evolving Universe12 What if. n
Conversations with alien intelligence requires patience! l disc of
Milky Way is ~40000 light years in radius optimistically suppose
1000 civilisations nearest one, on average, 2000 light years away
wait 4000 years for answer: not very practical so, dont converse,
just send Encyclopaedia Britannica and assume they will too
advantage: will also take 4000 years for their invasion fleet to
get here.. n Theyre not there because theyre not here? l unmanned
(unaliened?) probes could in principle colonise Galaxy fact that
this has not happened suggests no advanced civilisations? advanced
civilisations are all eco-warriors, robot probes environmentally
unfriendly?
Slide 13
Susan CartwrightOur Evolving Universe13 Conclusions n Evidence
from history of life on Earth is that origin of life may be easy l
evidence of past existence of life on Mars would be strong
confirmation of this n Basic criteria for stars suitable for life
are not too difficult to satisfy l detection of Earth-like planets
around nearby stars possible on 10-20 year timescale spectroscopy
could provide evidence for life on these planets n Other
technological civilisations might exist l probability depends on
hard-to-estimate factors l radio searches so far found nothing, but
you never know even if they do exist, United Federation of Planets
probably precluded by large distances (at least on basis of current
physics)